The present invention relates to the removal of nitrogen oxides from the exhaust gas of a lean-burn internal combustion engine and to an exhaust-gas purification system for performing the method. The method relies on the selective catalytic reduction (SCR) of nitrogen oxides using ammonia and on a partial upstream oxidation of nitrogen monoxide to nitrogen dioxide.
Observance of limits planned within the context of EU V in Europe and LEVII in the United States regarding pollutant emissions during operation of internal combustion engines can only be ensured by an active exhaust-gas aftertreatment and exhaust-gas purification, respectively. While the exhaust-gas purification in gasoline engines has largely been solved by the use of three-way catalytic converters, the particulate and nitrogen-oxide emissions from lean-burn internal combustion engines constitute the main problem. In order to convert the nitrogen oxides developed during fuel combustion, two different catalytic methods have been developed: one is the NOx adsorber technology in which, during lean operating states of the engine, nitrogen oxides are adsorbed at a suitable storage material and, at rich operating points, are desorbed and reduced; and the other is the SCR technology in which the nitrogen oxides are reduced using ammonia or a corresponding precursor compound convertible into ammonia.
While the sensitivity to sulfur and the required long-term stability represent two critical issues in the NOx adsorber technologies, the SCR method has in many cases already proved its suitability in long-term use for the removal of nitrogen oxides from power-station exhaust gases. In addition, it appears that according to the present state of the art the NOx conversion rates of partly up to 90% required in future can only be realized by employing the SCR method. Especially in heavy-duty trucks where an operating life of more than 400,000 miles is required SCR systems will very likely be employed.
Due to the high toxicity and volatility of ammonia, nontoxic precursor compounds are preferably used in motor traffic. In particular, aqueous urea solutions are used for this purpose. The urea solution is hydrolyzed to ammonia and carbon dioxide using hydrolysis catalysts or directly on the SCR catalyst. By means of special metering systems upstream of the hydrolysis and the SCR catalyst, respectively, the urea solution is injected or sprayed into the exhaust-gas flow.
The operating temperature of typical SCR catalysts on the basis of the solid acid systems V2O5/WO3/TiO2 and V2O5/MO3/TiO2, respectively, ranges between 300° C. and 550° C. In this range, they achieve nitrogen conversion rates of 90 to 100%. Likewise, the operating temperature of SCR catalysts on the basis of metal-ion exchanged zeolites mostly exceeds 300° C. depending on the metal ion. These catalysts are not very suitable for the conversion of nitrogen oxides at temperatures below 300° C.
The nitrogen oxides contained in the exhaust gas of internal combustion engines consist of 60 to 95 vol. % nitrogen monoxide depending on the operating state of the engine. It is known that the conversion of the nitrogen oxides can be improved in the SCR method if the exhaust gas contains approximately equal volume portions of nitrogen monoxide and nitrogen dioxide.
Therefore, in order to increase the “low-temperature activity” of SCR catalysts, a platinum-containing catalyst is generally arranged upstream of the urea injection site in the exhaust-gas line, which oxidizes part of the engine-generated nitrogen monoxide to nitrogen dioxide. In addition, this upstream oxidation catalyst, under appropriate conditions, almost completely oxidizes the hydrocarbons contained in the raw exhaust gas and prevents these hydrocarbons from diminishing the activity of the SCR catalyst by occupying its active centers. As a result, it is possible to remarkably expand the activity window of the SCR catalysts on the basis of the solid acid systems V2O5/WO3/TiO2 and V2O5/Mo3/Tio2, respectively, and on the basis of metal-ion exchanged zeolites. Generally, such systems achieve full conversion of the nitrogen oxides already from about 250° C.
However, in order to observe future limits, use of SCR systems for purifying the exhaust gases of passenger cars requires high nitrogen conversion rates in the temperature range as low as between 150 and 250° C. Catalysts having operating temperatures for the selective catalytic reduction below 200° C. have previously been described in the literature, for example in [R. M. Heck et al., Operating Characteristics and Commercial Operating Experience with High Temperature SCR NOx Catalyst, Environmental progress, 13 (1994) 4, pp. 221-225]. These are platinum-containing catalysts wherein highly dispersed platinum is present on a high surface area support. In the present invention, a high surface area support is generally meant to be a temperature-resistant metal oxide having a specific surface area of more than 10 m2/g. This includes, for example, the so-called active aluminum oxides having specific surface areas between 40 und 400 m2/g.
The range of operation of the platinum-containing catalysts for the selective catalytic reduction is limited towards high temperatures. That is, at temperatures above approximately 300° C., platinum starts oxidizing the ammonia to an increasingly greater extent, thereby removing it from the process of catalytic reduction.